This invention relates to a positive-displacement pump, namely to a piston pump of the type that is often used to pump liquid, such as water or oil, from a well by lifting or pushing up a liquid column accommodated in a delivery conduit and resting on the pump piston. Pumps of that type are often called lifting pumps.
More particularly, the invention relates to a positive-displacement pump of the kind defined in the precharacterising part of the independent claim.
At the lower end, in or beneath the pump chamber, that is, beneath the region within which the displacement member, the pump piston, moves up and down during the pumping operation, the classic lifting pumps have a stationary inlet valve in the form of a check valve which opens from the inlet into the pump chamber and an outlet valve carried by the pump piston, the outlet valve being a check valve which opens from the pump chamber into the delivery conduit.
In a variant of these pumps the pump piston carries the inlet valve and the outlet valve is placed at the top of the pump chamber or above it. An example of a lifting pump representing this variant is disclosed in WO90/08898.
The lifting member of the actuating mechanism may be a purely tensile member, i.e. a cable or some other flexible element which is essentially only capable of transmitting tensile forces and can therefore only effect the upward movement of the pump piston and limit the downward movement thereof. In such a case, the downward movement of the pump piston may be produced by the piston itself by virtue of its having a great weight and/or its being weighted to overcome the piston friction and the force required to open the valve. However, the lifting member may also be a rod or other stiff element that can also transmit compression forces to push down the pump piston.
The invention relates to a pump that can be categorised as being of the abovementioned variant and is improved in various respects over the prior art pumps. These improvements are achieved through the features set forth in the claims.
The pump according to the invention can be designed so as to be simple structurally and in production and thereby reliable and inexpensive. Although it is not so limited, it is therefore particularly suited for use as a well pump for pumping water in geographical regions where an inexpensive and easy-to-use and easy-to-install well pump is required in order that the population may have local access to well water.
The available cross-sectional area in the delivery conduit can be utilised to a large extent, meaning that the cross-sectional area of the pump piston can be a large percentage of the cross-sectional area of the delivery conduit. Even if the diameters of the wellbore and the delivery conduit are small, each pump stroke can yield a sufficiently large delivery volume without the stroke length having to be excessive.
Moreover, in the pump according to the invention, the pressure within the liquid column accommodated in the delivery conduit above the pump chamber can be utilised to bring about the downward movement of the pump piston or, more accurately, to add to the normally quite insufficient downward force resulting from the weight of the piston. A sufficient downward force can therefore be achieved without adding weight to the piston, even where the lifting member is a cable.
This effect is achieved because the displacement member is hydrostatically inbalanced such that it has an upper, i.e. upwardly facing thrust surface, which is always situated upstream of the displacement member during the movement of the displacement member and therefore constantly subjected to the pressure existing in the outlet (the delivery conduit), and a corresponding downwardly facing lower thrust surface, which is always situated in the inlet during the movement of the displacement member and thereby constantly subjected to the pressure existing in the inlet. During the pumping, the liquid column in the delivery conduit constantly causes the pressure in the outlet to be substantially greater than the pressure in the inlet, and because of the pressure differential that is thus always present between the outlet and the inlet, i.e. between the upper thrust surface and the lower thrust surface, the displacement member will constantly be subjected to a downward hydrostatic force, the magnitude of which is proportional to the pressure differential and the surface area of the net thrust surface.
Moreover, the pump piston and the inlet and outlet valves can readily be extracted as a unit to the ground level for inspection or maintenance, such as replacement of seals, and then reintroduced into the well. This operation does not require the delivery conduit to be extracted.
The delivery conduit can be a plastic hose which can easily be inserted in the well bore hole and extracted again when required. The hose can be cut on site to the length appropriate in each case, or it can be delivered in the desired length in the form of a coil by a local supplier. On installation of the pump, the pump housing is attached to one end of the hose and the hose is then slipped into the well bore hole to the desired depth, which may be, say, 100 m or more, without jointing being necessary. If the hose is coiled, it can uncoiled successively during the insertion in the wellbore. When the hose with the pump housing is in position, the pump piston and the components associated with it are brought down to the pump housing through the hose. Alternatively, the pump piston and the components associated with it can be placed in the pump housing prior to the insertion of the hose.